Human language is extremely complex and beyond the grasp of any other animal. How language is acquired by all normally developing preschool children is an outstanding scientific mystery. One goal of the proposed research aims to decrease this mystery by characterizing the prelinguistic representational capacities of human infants. The research focuses of two conceptual domains fundamental to natural language syntax: representation of number and representation of individual objects and kinds of objects. The research exploits several systematic infant behaviors in order to characterize the prelinguistic representations that underlie there. These behaviors include the deployment of attention and patterns of manual search. These methods yield evidence for rich representational capacities. They also yield evidence that infant nonlinguistic representations differ from linguistic representations in many respects. For example, the proposed studies will test the hypothesis that whereas human infants represent the precise numerosity of small sets of objects, they do so without nonlinguistic symbols for integers. For another example, the proposed studies test the hypothesis that representations for kinds of objects in the form that would support count nouns emerge between 10 and 12 months of age. Studies of word comprehension, word learning, and the acquisition of an explicit number list ("one, two, three. . .") will explore the role of language learning in early cognitive development. Studies of early cognition and language acquisition in normally developing children are crucial to understanding abnormal development. These studies have implications for the study of language disability and mental retardation, although these implications are not pursued in the present proposal.